The present invention relates to a novel process for producing a coagulated latex and apparatus therefor.
Conventially, in recovering polymer particles from a suspension liquid obtained by suspension polymerization or a high polymer latex obtained by emulsion polymerization (hereinafter, explanation will be made as to the high polymer latex), a coagulating agent such as inorganic salts, acids and the like is added to a latex, or conversely, a latex is added to an aqueous solution containing a coagulating agent and the latex is coagulated in an aqueous phase, converted to slurry through thermal treatment and the like, then subjected to dehydration and drying to thus obtain powdery polymer particles. The polymer particles, however, obtained by the process are irregular in shape, broad in particle size distribution and contains a considerably large quantities of fine powder. This causes many problems including a large expense for preventing loss of product resulting from escape of fine powder, frequent interruption of operation brought about by the plugging of passages by fine powder, contamination of the environmental atmosphere of the operating room by the escape of fine powder and the danger of explosion because of emitted fine powder. Moreover, due to the difficulty in raising bulk density of the polymer particles, expenses for transportation, storage and the like become costly. Furthermore the coagulated latex is inferior in dehydrative property and drying behavior so that an expensive dehydrating and drying equipment is necessitated to improve fluidity and anti-blocking property.
Recently, in order to overcome the deficiencies as aforesaid, a process is developed and disclosed in the Japanese Patent Non-examined Publication Nos. 3637/1977 and 30647/1978, in which coagulation and granulation of a high polymer latex is carried out simultaneously in a gaseous phase. According to the process, by dispersing a high polymer latex as droplets into a gasseous phase containing a coagulant (a gasseous coagulant or a mist of a coagulant solution), the latex is coagulated enough to hold the shape of coagulated particles formed in the gaseous phase even when hitting water or the organic solvent, then the coagulated particles are caused to enter or fall in the foregoing liquid phase to thereby be recovered. The process produces polymer particles of regular and substantially spherical shapes and of narrow particle size distribution and having good dehydrative property, drying behavior, fluidity, anti-blocking property and the like as well as large bulk density, and accordingly the foregoing problems such as loss of polymer particles, frequent operational troubles due to plugging, contamination of working environment and danger of powder explosion coming from a large amount of fine powder mingled, attendant on a process of production of a coagulated latex in a liquid phase as aforesaid are not only overcome, but reduction in cost of a dehydrating and drying equipment, utilities, transportation, storage and the like is resulted.
An apparatus for producing a coagulated latex in a gaseous phase is develoved and disclosed by the assignee of the present invention in the Japanese Patent Non-examined publication Nos. 33244/1978 and 137873/1978. An apparatus disclosed in No. 33244/1978 is comprised of a coagulating chamber, a means for spraying into the chamber a high polymer latex as droplets having an average particle size ranging from several microns and several millimeters, a means for dispersing in the chamber a coagulant to form a coagulating atmosphere, a receiving tank, located under the chamber, filled with a liquid in which the coagulated polymer particles are collected, and a means for removing from the receiving tank slurry containing the coagulated polymer particles.
In the apparatus as aforesaid, nonetheless, there are involved problems; the coagulated latex particles formed in the coagulating atmosphere deposit onto the inside walls of the coagulating chamber to thereby interrupt continuous operation over a long period of time and a part of the deposited latex particles peels off to mingle with the product, degrading the quality of product.
An apparatus revealed in No. 137873/1978 is developed to eliminate those defects, wherein, in the apparatus described in No. 33244/1978, a means for distributing a downward flow liquid which causes water or an organic solvent to flow downward on or along the inside walls of the coagulating chamber is provided, and further a means for contacting a gas and a liquid, if necessary, is provided at the bottom of the chamber. In this apparatus, the coagulated particles arrived at the inside walls of the chamber are washed down with the downward flow liquid and thus no coagulated particles deposit onto the inside walls of the coagulating chamber. Moreover, when steam, air and the like are introduced into the coagulating atmosphere to serve as a spraying medium of the high polymer latex or the coagulant, otherwise, a diluent gas is introduced therein for the adjustment of concentration of reactants in the coagulating atmosphere, the coagulated particles are likely to be entrained by exhaust gas, but in the apparatus the coagulated particles entrained by the exhaust gas are collected by bringing the downward flow liquid into contact with the exhaust gas through the gas-liquid contacting means, as a result, loss of coagulated particles is prevented and the coagulant and the like contained in the exhaust gas were removed to thereby prevent environmental contamination.
Notwithstanding, said apparatus described in No. 137873/1978 involves the following problems. That is, when a high polymer latex or a coagulant is introduced from a top of a coagulating chamber through a spraying device, atmosphere in the chamber is partly disturbed or a flow such as a rotating flow takes place. This phenomenon is vigorous when a two-fluid nozzle as a spraying device and steam or air as a dispersing medium are employed. Moreover when a centrifugal disc is used, a vertical air current occurs through a high-speed rotation of it. The vertical air current also occurs through the generation of steam, when a downward flow liquid of a high temperature is employed to raise the coagulation temperature. By the turbulence of the atmosphere, reactants and reaction products are whirled up at the top of the chamber to adhere to a top cover, a spraying device and the like, by which normal operation is difficult to continue and a long-period continuous operation is hindered. That is, said apparatus disclosed in No. 137873/1978 neither prevent effectively the occurrence of turbulent air current nor prevent reactants and reaction products from depositing onto the top cover, the spraying device and the like. In the apparatus, though exhaust is effected through a gas-liquid contacting means located at the bottom of the chamber, reacting atmosphere, especially a flow of atmosphere at the top of the chamber is not adequately rectified, and hence turbulence of atmosphere occurs at the top of the chamber, in particular, with a result that reactants and reaction products are not prevented from depositing onto the top cover and the spraying device. Moreover a downward flow liquid distributing means used in said apparatus is comprised of a ring-shaped pipe, located along the peripheral walls, having many holes directing to the coagulating chamber, by which whirling up of the reactants and the reaction products at the top of the chamber is not yet prevented. Furthermore, the apparatus described in No. 137873/1978 is so designed that the downward flow liquid having collected the reaction products is allowed to flow downward naturally from a removal outlet positioned in the air under the gas-liquid contacting means, but exhaust in the chamber is difficult owing to aspirating of the air into the chamber from said removed outlet, even if possible, a sucking means of a large capacity has to be used.
Besides the foregoings, said apparatus further involves the problems as set forth below. That is, in the apparatus a great amount of polymer scales is formed in and deposit, in a short period of time, only a supply pipe for supplying latex to the spraying device, which makes it impossible to supply latex in a desired amount per unit time to the spraying device. At the same time, the spraying of latex becomes unstable to thus prevent continuous operation for a long period of time--for four-day continuous operation, at longest.
Still worse, since the spraying device of the high polymer latex is located in the open coagulating atmosphere, the coagulating atmosphere intrudes into the spraying device, prior to supply of latex upon commencement of operation or at the time of stoppage of supplying latex upon suspension or termination of operation. Especially when head pressure of latex is not maintained, even the supply pipe is filled with the coagulating atmosphere. In such cases latex is coagulated to be solid therein by contact with the coagulating atmosphere, with which the spraying device and the supply pipe are choked up, continuous operation being thus disturbed.
Revealed in the Japanese Patent Application No. 139103/1980 is an apparatus in an attempt to solve such defects. That is, in the apparatus of production of coagulated latex comprising a coagulating chamber, a spraying means for dispersing a high polymer latex as droplets into the chamber, a means for forming a coagulating atmosphere in the chamber with which the latex droplets are coagulated, and a means for removing from the chamber slurry containing coagulated particles of high polymer latex, it is characterized in that a means for distributing downward flow liquid which forms a film-like flow of water or an organic solvent covering, at least, the greater part of the top of the coagulating chamber and causes it to flow downward along the side walls, after hitting it against the side walls, a means for rectifying the flow of atmosphere in the coagulating chamber by introducing air from the top of the chamber and forcing it to discharge from the bottom thereof, a gas-liquid contacting means for bringing the downward flow liquid into contact with the exhaust gas, a slurry removing line connected to the lower portion of the gas-liquid contacting means and a slurry receiving tank connected to the slurry removing line, said slurry removing line being connected airtight to said slurry receiving tank in such a manner that air may not flow backward in the slurry removing line by forced exhaust of said rectification means, and a supply pipe for supplying the high polymer latex to said spraying means having the inside finished surface of buff 200 or more and a diameter such that the shear stress on the inside surface of the pipe is 100 g/cm.multidot.sec.sup.2 or more.
The apparatus succeeded in solving the defects attendant on the apparatus of No. 137873/1978, there are still involved problems including coloration and thermal degradation resulting from oxidation of coagulated particles caused by introduction of air.
Moreover, as an important problem common to all conventional apparatus, at atmospheric prissure those are only applied to synthetic resins having Vicat softening point (hereinafter referred to "softening point") not higher than 100.degree. C. When those are applied to synthetic resins having softening point exceeding 100.degree. C., softening point has to be lowered beforehand by addition of a suitable solvent. That is because, when such high heat-resistant resins are subjected to the conventional apparatus, only soft and fragile spherical coagulated particles are obtained and those particles are destroyed by impact upon being collected in the downward flow liquid along the side walls, to be fine powder of irregular shapes.
In application of those high heat-resistant resins to the conventional apparatus, the system is considered to be placed under an increased pressure to keep the temperature of the coagulating chamber at the vicinity of high softening point, but it is not advisable in practice because of many difficulties on equipment including an increase in cost for remodeling it to a pressure-resisting apparatus and high technology required for a removal-supply system.